1. Field of the Invention
The present invention relates to a sintered polycrystalline diamond composite for use in rock drilling, machining of wear resistant metals, and other operations which require the high abrasion resistance or wear resistance of a diamond surface. Specifically, this invention relates to polycrystalline diamond which has been sintered at high temperatures- and high pressure with the aid of a catalyst to form strong diamond to diamond bonding.
2. Description of the Art
Composite polycrystalline diamond compacts or PCD have been used for industrial applications including rock drilling and metal machining for many years. One of the factors limiting the success of the PCD is the generation of heat due to friction between the PCD and the work material. This heat causes thermal damage to the PCD in the form of cracks which lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear.
When the PCD cutter is new, it generally has a circular geometry, and it thus presents a sharp cutting edge to the work material. However, after use for some time, this circular or arc-shaped cutting edge wears into a straight flat surface that cannot as effectively penetrate the work material. When used for rock drilling, the worn PCD cutter acts as a friction bearing surface that generates heat, which accelerates the wear of the PCD cutter and slows the penetration rate of the drill. FIG. 1 shows a wear flat generated on a prior art cutter at location 1.
Prior art methods to solve this problem, such as discussed in U.S. Pat. No. 4,784,023 to Dennis, utilize a substrate with a non-planar surface so that the interface between the diamond and the substrate is irregular. The result is a diamond layer which has both thin and thick sections. The thicker portion of the polycrystalline diamond offers more abrasion resistance and wears at a slower rate. Failure analysis of drill bits containing PCDs with non-planar interfaces shows that the worn cutting edges of the cutters are irregular and sharper than those of cutters made with planar interfaces. Although this has generally been shown to be an improvement, there is still an area of concern. When non-planar substrates are used, highly localized stress occurs at the interface, causing cracking which can result in catastrophic failure of the cutter.
In U.S. Pat. No. 4,784,023, the disadvantage of using relatively few parallel grooves with planar side walls is that the stress becomes concentrated along the top and, more importantly, the base of each groove and results in significant cracking of the metallic substrate along the edges of the bottom of the groove. This cracking significantly weakens the substrate whose main purpose is to provide mechanical strength to the thin polycrystalline diamond layer. As a result, construction of a polycrystalline diamond cutter following the teachings provided by U.S. Pat. No. 4,784,023 is not suitable for cutting applications where repeated high impact forces are encountered, such as in percussive drilling; nor is it suitable in applications where extreme thermal shock is a consideration. FIG. 2 shows a prior art cutter of this design and the location of cracking at edges 3.
Other configurations have been proposed in order to overcome problems of stress in the compact due to the mismatch in thermal expansion between the diamond layer and the tungsten carbide substrate. For example, U.S. Pat. No. 5,351,772 describes the use of radially extending raised lands on one side of the tungsten carbide substrate area on which a polycrystalline diamond table is formed and bonded.
U.S. Pat. No. 5,011,616 describes a substrate with a surface topography formed by irregularities having non-planar side walls, such that the concentration of substrate material continuously and gradually decreases at deeper penetrations into the diamond layer. U.S. Pat. No. 5,379,854 describes a substrate with a hemispherical interface between the diamond layer and the substrate, the hemispherical interface containing ridges that penetrate into the diamond layer. U.S. Pat. No. 5,355,969 describes an interface between the substrate and polycrystalline layer defined by a surface topography with radially-spaced-apart protuberances and depressions.
All of the above proposals show a diamond layer of varying thickness relative to the surface of the tungsten carbide substrate support. Thus, in areas where the diamond layer is thicker, the amount of cobalt available is less than in those areas where the diamond layer is thin. This results in a non-uniformly sintered diamond layer that substantially weakens the compact. Even when cobalt powder is pre-mixed with the diamond prior to subjecting the compact to high pressure-high temperature conditions, the presence of cobalt in a substrate with a textured surface produces areas of varying concentration of cobalt within the diamond layer during the sintering process and causes soft spots or poorly sintered areas within the diamond layer.
It has been proposed to use transitional layers to better sinter the diamond and improve the adhesion of the polycrystalline diamond to the substrate.
One of the solutions to these problems is proposed in U.S. Pat. No. 4,604,106. This PCD utilizes one or more transitional layers incorporating powdered mixtures with various percentages of diamond, tungsten, carbide, and cobalt to distribute the stress caused by the difference in thermal expansion over a larger area. A problem with this solution is that the cobalt cemented carbide in the mixture weakens that portion of the diamond layer because less diamond-to-diamond direct bonding occurs as a result of the carbide second phase.
U.S. Pat. No. 4,311,490 teaches the use of coarse diamond particles next to the tungsten support with a layer of finer diamond particles placed on top as the exposed cutting surface. This is reported to reduce the occurrence of soft spots or poorly sintered areas in the diamond table since the coarser particles have larger channels between them, making it easier for cobalt to sweep through the diamond nearest the tungsten carbide substrate, thus allowing thicker diamond layers to be sintered. For rock drilling applications, however, it has been found that although finer diamond results in higher abrasion resistance, it also results in significantly less impact resistance. The lower impact resistance produces compact cutter failure by way of fracturing and spalling of the diamond layer from the tungsten carbide support substrate.
U.S. Pat. No. 5,645,617 also uses layers of diamond with different average particle sizes.
The problem with the layer designs is that they do not provide a means to cause irregular wear of the cutting edge and thus do not eliminate the problem of formations of a relatively large wear flat. Thus, it would be useful to have a means to control the geometry of the cutting edge and at the same time limit the stress caused by using non-planar interfaces.
U.S. Pat. No. 5,855,996 shows a polycrystalline diamond compact which incorporates different sized diamond. Specifically, it describes mixing submicron sized diamond particles together with larger sized diamond particles in order to create a more densely packed compact by filling the interstices between the larger diamond with diamond particles of smaller size. The problem with this approach is that the uniformly dense diamond PCD produces a uniformly flat, dull working surface at the cutting interface. The dull edge thus created as the PCD wears slows the penetration rate in rock drilling applications, creates heat, and cracks once initiated proceed unhindered across the entire PCD layer.
U.S. Pat. No. 6,187,068 B1 teaches the separation of diamond into laterally spaced regions of discrete particle size areas. The polycrystalline diamond areas formed of the finer size diamond particles provide a higher abrasion resistance and a slower wear rate, thus providing a non-linear cutting edge. The problem with this solution is that the areas of different diamond size are relatively large in size and do not provide a sharp enough cutting edge as wear progresses. Thus, although offering an improvement over other geometries, heat is still generated at the working surface. Additionally, the large geometric patterns of differing particle sizes produce stress along the boundaries between the discrete particle size areas leading to catastrophic fracture of the polycrystalline material. FIG. 3 shows a cross section of a prior art cutter of this design. Cracking occurs at the edge boundaries 4 and 5 between the different geographic areas.